Process for the preparation of polyurethane resins and their use and the use of ethoxyethyl propionate for the preparation of polyurethane resins

ABSTRACT

The present invention relates to a process for the preparation of polyurethane resins, in which, in an ester as solvent, 
     a) at least one compound having 2 hydroxyl and/or amino groups, 
     b) at least one di- and/or polyisocyanate, 
     c) at least one compound containing 2 groups which are reactive toward isocyanate groups, these compounds containing, at least in part, at least one group capable of forming anions which is neutralized before or after incorporation of component (c) into the polyurethane molecule, and 
     d) at least one compound having at least 3 hydroxyl and/or amino groups 
     are used to prepare a polyurethane resin, preferably using components (a), (b) and (c) to prepare an intermediate which has terminal isocyanate groups and at least some of whose free isocyanate groups are subsequently reacted with component (d), characterized in that ethoxyethyl propionate is employed as solvent.

The present invention relates to a process for the preparation ofpolyurethane resins, in which, in an ester as solvent,

a) at least one compound having 2 hydroxyl and/or amino groups,

b) at least one di- and/or polyisocyanate,

c) at least one compound containing 2 groups which are reactive towardisocyanate groups, these compounds containing, at least in part, atleast one group capable of forming anions which is neutralized before orafter incorporation of component (1 c) into the polyurethane molecule,and

d) at least one compound having at least 3 hydroxyl and/or amino groupsare used to prepare a polyurethane resin, preferably using components(a), (b) and (c) to prepare an intermediate which has terminalisocyanate groups and at least some of whose free isocyanate groups aresubsequently reacted with component (d).

The present invention also relates to the use of the polyurethane resinsand to the use of ethoxyethyl propionate as solvent for the preparationof polyurethane resins.

The present invention consequently also relates to coating compositionscontaining these polyurethane resins, processes for the production ofthese coating compositions and processes for coating substrates usingthese coating compositions.

Aqueous polyurethane dispersions and processes for their preparation areknown to those skilled in the art. Of particular technical importance isthe so-called acetone process. In this process, generally, a prepolymerhaving NCO end groups is first prepared, which is subsequently dissolvedin an inert solvent followed if appropriate by chain lengthening insolution to give a polyurethane of higher molecular weight. Theadditions made in the so-called acetone process of low-boiling solventssuch as, for example, acetone are necessary inter alia in order toreduce the viscosity of the prepolymer and thus to render it capable ofbeing handled, subsequent dispersion only then being possible.

The disadvantage of such preparation processes, taking into account theusually applicable requirement for solvent-free products, is that atechnically complex distillation step has to follow the dispersionprocedure in order to remove—at least predominantly—the low-boilingcomponent such as acetone. Ketones, for example acetone, are in fact acause of poor coatings properties in an aqueous system. In addition,ketones such as acetone are toxic to fish and are notable for relativelylow flash points (acetone −18° C., methyl ethyl ketone −4.4° C. andmethyl isobutyl ketone +15.5° C.). In the USA methyl ethyl ketone andmethyl isobutyl ketone are on the list of hazardous air-pollutingsubstances, so that even residual quantities in the coating should beavoided.

Replacing ketones by other solvents is carried out in a strippingprocedure. This procedure involves removing the ketone as a mixture withthe second solvent in vacuo. This implies an additional process stepwhich not only complicates the process but also leads to an increase inthe cost of the product. The latter not least because the acetonepreferably employed cannot be recycled directly to the process, sinceanhydrous acetone is preferably employed. The question for the personskilled in the art is thus whether and, if so, to what extent a residualsolvent content is acceptable, since it is on this that the complexityof the process depends. To fulfill the requirement for an entirelysolvent-free product prepared by this process necessitates acomparatively high expenditure.

The desire is therefore for solvent substitutes which can be employed,instead of the solvents hitherto known and used, in the preparation ofaqueous polyurethane dispersions. In this context the solvents used asan alternative to ketones must meet a number of requirements:

ignition temperatures >200° C.

inert toward isocyanates (including any impurities)

widely available industrially

adequate miscibility with water (may possibly be improved by cosolvents)

flash point >55° C.

suitable for coatings (ability to be evaporated)

favorable price

appropriate boiling point.

In addition to ketones, it is in principle also possible to employethers and esters for the synthesis of polyurethane resins. Thus DE-A-4013 546 discloses the employment of propanediols and/or triols esterifiedwith acetic acid as solvents for the preparation of polyurethanedispersions by processes which are known per se.

The use of these solvents described in DE-A-40 13 546, however, has thedisadvantage that esters of acetic acid, such as methoxypropyl acetate,ethoxypropyl acetate and butoxypropyl acetate, are in generalsusceptible to hydrolysis. In addition they are not suitable for thepreparation of polyurethanes at elevated temperatures of from 100 to130° C. since even at these temperatures they undergo considerabledecomposition in the reaction mixture. However, because of ever stricterenvironmental regulations the requirement is for polyurethane resinsolutions with as high a solids content as possible but the preparationof such solutions necessitates the use of elevated temperatures in orderto overcome the viscosity problems. In addition, higher temperatures aredesirable because of the reduction in reaction time which is associatedwith them.

Also unsuitable in practice is, for example, methoxypropyl acetate,since at 50 ppm it has too low an MAC value (maximum workplaceconcentration). Furthermore, methoxypropyl acetate is classified inpregnancy group C. Ethoxypropyl acetate has the disadvantage that theresulting polyurethane solutions are of very high viscosity.Butoxypropyl acetate, finally, has to be ruled out because of its poorsolubility in water. DE-A 41 10 520 discloses the preparation ofpolyurethane resins in solvents which remain in the coating material anddo not have to be exchanged. Examples of suitable solvents which arespecified are methoxypropyl acetate, ethoxyethyl acetate (common namefor ethylene glycol monoethyl ether acetate) and N-methylpyrroline. Theacetates specified, however, have the disadvantages already indicated.Moreover, ethoxyethyl acetate has the disadvantage that this solvent,because of the regulations which apply, cannot be used in the USA, forexample. These acetates mentioned in DE-A 41 10 520 are thereforelikewise not used in practice. Finally, N-methylpyrrolidone (NMP) hasthe disadvantage that it does not evaporate from the coating film,especially at low temperatures. In addition, the resins prepared in NMPhave higher viscosities in comparison with the resins prepared in EEP.The color number of the resins prepared in NMP is, moreover, higher.

The object of the present invention is therefore to provide a processfor the preparation of polyurethane resins which permits thecost-effective preparation of polyurethane dispersions and does notinvolve a stripping procedure. This process should also enable thepreparation of polyurethane resin solutions having a high solidscontent. Furthermore, the polyurethane dispersions prepared by thisprocess, when employed in aqueous coating compositions, should lead tocoatings whose mechanical properties conform as closely as possible tothe properties of coatings obtained using polyurethane dispersions whichhave been prepared conventionally.

This object is surprisingly achieved by a process of the type mentionedat the outset, which is characterized in that the polyurethane resin isprepared in ethoxyethyl propionate (EEP) as solvent.

The present invention also relates to the use of the polyurethane resinsand to the use of ethoxyethyl propionate as solvent for the preparationof polyurethane resins. In addition, the invention also relates tocoating compositions containing these polyurethane resins, to processesfor the production of these coating compositions and to methods ofcoating substrates using these coating compositions, and to substratescoated with these coating compositions.

It is surprising and was not foreseeable that the use, specifically, ofethoxyethyl propionate as solvent for the preparation of polyurethanedispersions in processes known per se would enable the cost-effectivepreparation of polyurethane dispersions in a process not involving astripping procedure. Ethoxyethyl propionate is commercially availableand obtainable directly in relatively large quantities. A furtheradvantage is that, when ethoxyethyl propionate is used as solvent,because of its good viscosity-regulating properties and its goodstability at elevated temperatures, it is also possible to preparepolyurethane resin solutions with high solids contents. The polyurethaneresin solutions prepared by the process according to the invention arestable on storage over a long period of at least approximately 12months. In addition, the use of ethoxyethyl propionate as solvent in theknown processes for the preparation of polyurethane dispersions has theadvantage that the other process parameters do not require significantalteration. Finally, coatings produced using the polyurethanedispersions prepared in accordance with the invention have goodmechanical properties, which are comparable with the properties ofcoatings obtained using polyurethane dispersions which are preparedconventionally.

The process according to the invention is explained in more detailbelow.

It is essential to the invention that, in the processes known per se forthe preparation of polyurethane dispersions, ethoxyethyl propionate isused as solvent instead of the solvents conventionally employed.Ethoxyethyl propionate (also called ethyl 3-ethoxypropionate [sic]) isoutstandingly suitable in this context as a replacement for these knownsolvents which have hitherto been customary. Ethoxyethyl propionate isunreactive with respect to the starting materials employed in thepreparation of the polyurethane resins, and the resulting polyurethaneresin solutions are stable on storage over a long period (at least 12months). In addition, ethoxyethyl propionate is commercially availableand is obtainable directly in relatively large quantities. Whenethoxyethyl propionate is used as solvent instead of the solventshitherto customary, it is in addition unnecessary substantially to alterthe other process parameters. Switching to the process according to theinvention can thus be carried out simply and rapidly and in acost-effective manner.

In some circumstances it may be necessary to operate the processaccording to the invention at an elevated reaction solids content (i.e.solvent content during the preparation of the polyurethane resinpreferably less than 20% by weight, based on the solids content) and/orwith an increase in the proportion of modifying agent and/or an increasein the reaction temperature (particularly preferred range from 100 to130° C.), in order to ensure that the number-average molecular weight ofthe polyurethane resin obtained corresponds to the number-averagemolecular weight of the resin prepared in a conventional manner.However, this can readily be determined on the basis of a few routineexperiments.

The starting substances which are suitable for the preparation of thepolyurethane resin solutions or polyurethane dispersions, for examplepolyols, isocyanates, chain extenders, reactive components capable offorming salts, and other auxiliaries, are known and are described, forexample, in the following publications:

DE-A 26 24 442, DE-A 32 10 051, EP-A 355 433, DE-A 35 45 618, DE-A 38 13866, DE-A 40 05 961, DE-A 41 10 520 and DE-A 40 13 546. In respect ofexamples of suitable structural components of the polyurethane resins,reference is therefore made to these publications.

In the process according to the invention for the preparation of thepolyurethane resin, it is preferred first to prepare an isocyanategroup-containing prepolymer which is then reacted further, preferably bychain lengthening, to prepare the desired polyurethane resin. In thiscontext components (a), (b) and (c) are reacted by the well-knownmethods of organic chemistry (cf. e.g. Kunststoff-Handbuch, [PlasticsHandbook], Volume 7: Polyurethane [Polyurethanes], edited by Dr. Y.Oertel, Karl-Hanser-Verlag, Munich, Vienna 1983), preferably by reactingthe components in stages (e.g. formation of a first intermediate fromcomponents (a) and (b) which is then reacted with (c) to give a secondintermediate). However, it is also possible to react components (a), (b)and (c) simultaneously. Examples of the preparation of the prepolymersare described in DE-A 26 24 442 and DE-A 32 10 051. In addition,however, the polyurethane resins can also be prepared by simultaneouslyreacting components (a), (b), (c) and (d).

The reaction temperature during the preparation of the prepolymer fromcomponents (a), (b) and (c) is usually up to 150° C., preferably between80 and 150° C. and particularly preferably from 100 to 120° C. Higherreaction temperatures, from 100 to 120° C., lead in this case to areduction in the reaction time and are also desirable for thepreparation of polyurethane resin solutions of high solids content,since this makes the resin solutions easier to handle (reduction inviscosity).

Components (a), (b) and (c) are reacted in ethoxyethyl propionate assolvent. In this context the quantity of ethoxyethyl propionate can bevaried within wide limits and should be sufficient to form a prepolymersolution of appropriate viscosity. In general up to 70% by weight,preferably from 5 to 50% by weight and particularly preferably less than20% by weight of solvent, based on the solids content, are employed.Thus the reaction can be carried out in a very particularly preferredmanner, for example, at a solvent content of 10-15% by weight of EEP,based on the solids content. Components (a), (b) and (c) can if desiredbe reacted in the presence of a catalyst, such as organotin compoundsand/or tertiary amines.

For the preparation of the prepolymers, the quantities of components(a), (b) and (c) are selected such that the ratio of equivalents of NCOto OH groups is between 2.0:1.0 and >1.0:1.0, preferably between 1.4:1and 1.1:1.

The NCO prepolymer contains at least approximately 0.5% by weight ofisocyanate groups, preferably at least 1% by weight of NCO, based on thesolids. The upper limit is at approximately 15% by weight, preferably10% by weight and particularly preferably 5% by weight of NCO.

The polyols (component (a)) employed for the preparation of theprepolymer may be of low and/or high molecular weight and may containanionic groups which are slow to react. To increase the hardness of thepolyurethane, low molecular weight polyols can be employed. They have amolecular weight of from 60 to approximately 400 and may containaliphatic, alicyclic or aromatic groups. In this case quantities of upto 30% by weight of the overall polyol components, preferably fromapproximately 2 to 20% by weight, are employed.

To obtain an NCO prepolymer of great flexibility, a high proportion of apredominantly linear polyol having a preferred OH number of from 30 to150 mg of KOH/g should be added. Up to 97% by weight of the entirepolyol may comprise saturated and unsaturated polyesters and/orpolyethers having a number-average molecular mass Mn of from 400 to5000. The polyether diols selected should not introduce excessivequantities of ether groups, since otherwise the polymers formed swell inwater. Polyester diols are prepared by esterifying organic dicarboxylicacids or their anhydrides with organic diols, or are derived from ahydroxycarboxylic acid or from a lactone. To prepare branched polyesterpolyols, it is possible to a minor extent to employ polyols ofpolycarboxylic acids having a higher functionality. Linear polyesterdiols are preferably employed.

Examples which can be mentioned of polyether polyols are polyoxyalkylenepolyols, especially polyoxypropylene glycols having a molecular weightof from 300 to 3000 (number average).

As component (a) it is also possible to employ polyester polyols whoseacid component consists at least partly of dimeric fatty acids. Suchsystems are described in, for example, U.S. Pat. No. 4,423,179. Apartfrom the diols listed, compounds containing amino groups are alsosuitable as component (a) for preparing the prepolymer, but preferablythe di- and polyols listed are employed.

Typical multifunctional isocyanates (component (b)) which are used arealiphatic, cycloaliphatic and/or aromatic polyisocyanates having atleast 2 isocyanate groups per molecule. The isomeric [sic] or isomericmixtures of organic diisocyanates are preferred. Because of their goodresistance to ultraviolet light, (cyclo)aliphatic diisocyanates resultin products with little tendency to yellowing. The polyisocyanatecomponent used to form the prepolymer may also contain a proportion ofmore highly functional polyisocyanates, provided that this does notcause any gel formation. Triisocyanates which have proven suitable areproducts formed by trimerization or oligomerization of diisocyanates orby reaction of diisocyanates with polyfunctional compounds containing OHor NH groups. The average functionality can be lowered if desired byadding monoisocyanates.

For the preparation of polyurethane resin solutions of high solidscontent, the diisocyanates employed are in particular those of thegeneral formula (I)

in which X is a divalent, aromatic hydrocarbon radical, preferably anoptionally halogen-, methyl- or methoxy-substituted naphthalene,diphenylene, 1,2-, 1,3- or 1,4-phenylene radical, particularlypreferably a 1,3-phenylene radical, and R₁ [sic] and R₂ [sic] are analkyl radical having 1 to 4 carbon atoms, particularly preferably amethyl radical.

Diisocyanates of the formula (I) are known (they are described in, forexample, EP-A 101 832, U.S. Pat. No. 3,290,350, U.S. Pat. No. 4,130,577and U.S. Pat. No. 4,439,616) and some are commercially available (forexample, 1,3-bis(2-isocyanatoprop-2-yl)benzene is sold by the AmericanCynamid [sic] Company under the tradename TMXDI (META)®).

In addition to the diisocyanates of the formula (I) or instead of them,it is also possible to employ other aliphatic and/or cycloaliphaticand/or aromatic polyisocyanates. Examples of polyisocyanates which canbe employed in addition are 1,3- and 1,4-phenylene diisocyanate, 2,4-and 2,6-tolylene diisocyanate, xylylene diisocyanate, 4,4′-bisphenylenediisocyanate, 1,4- and 1,5-naphthylene diisocyanate, diphenylmethanediisocyanate, isophorone diisocyanate, cyclopentylene diisocyanate,cyclohexylene diisocyanate, methylcyclohexylene diisocyanate,dicyclohexylmethane diisocyanate, trimethylene diisocyanate,tetramethylene diisocyanate, pentamethylene diisocyanate, hexamethylenediisocyanate, propylene diisocyanate, ethylethylene diisocyanate andtrimethylhexane diisocyanate.

Polyurethanes are generally incompatible with water unless specificcomponents have been incorporated and/or special preparation stepscarried out during their synthesis. For instance, the preparation of thepolyurethane resins can be carried out using compounds which contain twoH-active groups which react with isocyanate groups, and contain at leastone group which ensures dispersibility in water (carrier groups).Suitable carrier groups are nonionic groups (e.g. polyethers), anionicgroups, mixtures of these two groups, or cationic groups.

Thus the acid number incorporated into the polyurethane resin may be ofsuch an extent that the neutralized product gives a stable dispersion inwater. Compounds used for this purpose are those containing two H-activegroups which react with isocyanate groups, and at least one groupcapable of forming anions. Suitable groups which react with isocyanategroups are, in particular, hydroxyl groups and primary and/or secondaryamino groups. Groups capable of forming anions are carboxyl, sulfoand/or phosphonic acid groups. It is preferred to use carboxylic acid orcarboxylate groups. Their propensity to react should be so low that theisocyanate groups of the diisocyanate react preferentially with theother groups of the molecule which are reactive toward isocyanategroups. Alkanoic acids having two substituents on the a carbon atom areemployed for this purpose. The substituent may be a hydroxyl group, analkyl group or an alkylol group. These polyols have at least one,generally from 1 to 3 carboxyl groups per molecule. They have from 2 toabout 25, preferably from 3 to 10 carbon atoms. The carboxylgroup-containing polyol may make up from 3 to 100% by weight, preferablyfrom 5 to 50% by weight, of the overall polyol component in the NCOprepolymer.

The quantity of ionizable carboxyl groups available in salt form byneutralization of the carboxyl groups is in general at least 0.4% byweight, preferably at least 0.7% by weight, based on the solids. Theupper limit is about 6% by weight. The quantity of dihydroxy alkanoicacids in the unneutralized prepolymer gives an acid number of at least5, preferably at least 10. In the case of very low acid numbers, furthermeasures are generally necessary in order to achieve dispersibility inwater.

The upper limit for the acid number is at 70, preferably at 40, mg ofKOH/g, based on the solids. The acid number is preferably in the rangefrom 20 to 30 mg of KOH/g.

Examples of a suitable compound containing at least 2 groups which reactwith isocyanate groups and at least one group capable of forming anionsare dihydroxypropionic acid, dimethylolpropionic acid, dihydroxysuccinicacid or dihydroxybenzoic acid. Also suitable are the polyhydroxy acidsaccessible by oxidation of monosaccharin, for example glycolic acid,saccharic acid, mucic acid, glycuronic acid and the like.

Examples of compounds containing amino groups are α,δ-diaminovalericacid, 3,4-diaminobenzoic acid, 2,4-di-aminotoluene-5-sulfonic acid,4,4′-diaminodiphenyl ethersulphonic acid and the like.

Examples of tertiary amines which are suitable for neutralizing theanionic groups are trimethylamine, triethylamine, dimethylaniline,diethylaniline, triphenylamine, dimethylethanolamine and the like. Ifthe neutralization takes place in the organic phase, it is preferred toemploy triethylamine, with dimethylethanolamine being preferred in thecase of neutralization in the aqueous phase. As compounds which containtwo groups which are reactive toward isocyanate groups, but which arefree from groups capable of forming anions, it is for example possibleto employ low molecular weight diols or diamines containing primary orsecondary amino groups.

In a second stage, the isocyanate groups of the prepolymer which arestill present are reacted with a modifying agent. This reaction leads,in particular, to a further linking and to an increase in the molecularweight. The quantity of this modifying agent is determined by itsfunctionality and the NCO content of the prepolymer. The ratio ofequivalents of the active hydrogen atoms in the modifying agent to theNCO groups in the prepolymer should generally be lower than 4:1 andshould preferably be in the range between 3:1 and 2:1.

The modifying agents employed for the reaction with the prepolymer arepreferably diols and particularly preferably tri- and/or polyols.

However, it is also possible to employ other compounds containing activehydrogen atoms as modifying agents, for example polyamines, albeit onlyprovided that the reaction of the prepolymer with the modifying agentcan be carried out in an organic solvent (controllably) and that thisreaction is not accompanied by any unwanted reactions, for example thegelation at the point of dropwise addition of the amine, as is oftenobserved when using polyamines.

Examples of polyols containing at least 3 hydroxyl groups aretrimethylol propane, glycerol, diglycerol, erythritol, mesoerythritol,arabitol, adonitol, etc. Trimethylolpropane is preferably employed. Thereaction of the prepolymer with the tri- and/or polyol is preferablycontrolled by the stoichiometry of the compounds employed such thatchain lengthening occurs.

The polyurethane resins prepared by means of the process according tothe invention conventionally have a number-average molecular weight offrom 1000 to 30,000, preferably from 1500 to 20,000 (determined in eachcase by gel permeation chromatography with polystyrene as standard), andan acid number of from 5 to 70 mg of KOH/g, preferably from 10 to 30 mgof KOH/g.

The polyurethane resins prepared by the process according to theinvention and dissolved in ethoxyethyl propionate are usually dilutedfurther with other solvents, so that the polyurethane resin solutionsobtained preferably have a solids content of up to 70% by weight,particularly preferably a solids content of from 50 to 60% by weight.If, however, relatively large quantities of ethoxyethyl propionate areemployed during the actual preparation of the polyurethane resins, thendilution with other organic solvents can also be dispensed with.

Examples of solvents which are suitable for diluting the polyurethaneresin solution are butoxypropanol, methoxybutanol, propylene glycolmonopropyl ether, propylene glycol monoisopropyl ether, methyldiglycoland propylene glycol mono-tert-butyl ether. The selection of thesesolvents depends on the desired properties of the coating compositions.

The polyurethane resin solutions prepared by the process according tothe invention, prepared in ethoxyethyl propionate and, if desired,diluted further can be employed directly for producing the base colorsof a mixing system. In this case the further components of the basecolors, for example pigments, conventional auxiliaries and additivesand, if desired, other additional binders, and the like, areincorporated into the resulting polyurethane resin solutions—asdescribed in DE-A 41 10 520 —by methods which are known to those skilledin the art, by mixing and, if desired, dispersion.

For the production of water-dilutable coating compositions using themixing system, the water-free base colors required for the desiredcolor, and at least one water-containing, pigment-free component, arethen mixed shortly prior to the application of the coating composition.

The polyurethane resin solutions prepared by the process according tothe invention and containing ethoxyethyl propionate are also suitablefor the production of aqueous coating compositions. In this case thepolyurethane resin solution prepared by the process according to theinvention and containing ethoxyethyl propionate is transferred to anaqueous phase, which involves the solution being dispersed in water bymethods which are well known to those skilled in the art. In contrast tothe polyurethane resin solutions prepared by conventional processes, itis not necessary to distill off the ethoxyethyl propionate used assolvent. Instead, the ethoxyethyl propionate can remain in the coatingcomposition. It may if desired be advantageous also to add othercosolvents to the aqueous coating composition obtained. Butoxy propanolis particularly suitable for this purpose since, with butoxy propanol ascosolvent, excellent application results and outstanding optical andmechanical properties of the resulting coating are obtained. Inaddition—depending on the intended use and profile of requirements ofthe coating compositions—other solvents are also suitable as cosolvents,for example 3-methoxy butanol, propylene glycol monopropyl ether,propylene glycol mono-tert-butyl ether, propylene glycol monoisopropylether and butylglycol.

For the production of aqueous coating compositions, however, it is alsopossible first to dilute the polyurethane resins prepared by the processaccording to the invention and dissolved in ethoxyethyl propionate,further with the abovementioned cosolvents and then to disperse thesedilute polyurethane resin solutions in water by methods well known tothose skilled in the art. In this way, depending on the cosolventschosen, dispersion in water may in some cases be possible with greaterease.

The production of the aqueous coating compositions from the polyurethanedispersions prepared in accordance with the invention, by incorporatingthe remaining components such as, for example, pigments, auxiliaries andadditives, other binders and the like, is carried out by well-knownconventional methods (mixing and, if desired, dispersion) and thereforerequires no more detailed explanation.

The polyurethane resins prepared by the process according to theinvention are particularly suitable for use in coating compositions usedto produce multilayer coatings.

The present invention therefore also relates to a process for theproduction of a multilayer protective and/or decorative coating on asubstrate surface, in which

1.) a basecoat composition is applied,

2.) a polymer film is formed on the surface from the composition appliedin stage (1),

3.) a transparent topcoat is applied to the resulting basecoat, and

4.) the topcoat is cured together with the basecoat.

The process is characterized in that the basecoat composition applied instage (1) and/or the topcoat applied in stage (3) contains apolyurethane resin prepared by the process according to the invention.

The aqueous polyurethane dispersions prepared by the process accordingto the invention are particularly suitable, furthermore, for theproduction of aqueous coating compositions for the coating of motorvehicle bodies (production line coating) and/or plastic parts. However,they can also be applied to other substrates, for example glass, metals,wood and the like. In addition, they are preferably employed for theproduction of water-dilutable refinish coatings, especially in the areaof automotive refinishing. They are particularly well suited to theproduction of is aqueous basecoats for the refinishing of motorvehicles. In addition to this, however, the polyurethane dispersionsprepared in accordance with the invention also find numerous other areasof application, from adhesives and leather finishing compositions tocoating compositions for a very wide variety of application areas, forexample fillers or coating compositions for industrial products, largemachines etc.

The invention is now illustrated in more detail on the basis ofexamples. In these examples all indications of parts and percentages areby weight, unless expressly stated otherwise.

POLYESTER A

In an apparatus conventional for polyester synthesis, 891.2 parts ofPripol 1013 (commercial dimeric fatty acid with a monomer content of notmore than 0.1%, a trimer content of not more than 2%, an acid number offrom 195 to 198 mg of KOH/g and a hydrolysis number of 198-202 mg ofKOH/g), 292.8 parts of hexane-1,6-diol, 360.3 parts of isophthalic acidand 250.7 parts of neopentyl glycol together with xylene as entrainingagent are reacted to an acid number <5.0. The xylene is distilled offand the polyester is run up to an acid number of 3-4. The batch iscooled to 110° C. and dissolved in ethylethoxy propionate to a solidscontent of 70.2%. The polyester A has an Mn value of 2333 and an M_(w)value of 4912.

POLYESTER B

In an apparatus conventional for polyester synthesis, 371.2 parts ofPripol 1013, 107.7 parts of cyclohexanedimethanol, 723.3 parts ofneopentylglycol hydroxypivalate, 17.2 parts of ethylbutylpropanediol,392.6 parts of neopentylglycol, 0.8 part of n-butyltin oxide hydrate and1018.7 parts of isophthalic acid together with cyclohexane as entrainingagent are reacted to an acid number <5.0. The cyclohexane is distilledof f and the polyester is run up to an acid number of <1.5. The batch iscooled to approximately 110° C. and dissolved with ethylethoxypropionate to 79.5%. The polyester B has an M_(n) value of 2352 and anM_(w) value of 4578.

POLYESTER C

This polyester corresponds to polyester B, but has been dissolved to anSC of 79.5% in N-methylpyrrolidone.

EXAMPLES 1a, 1b, 1c and 1d Preparation of the Polyurethane ResinSolutions 1a-1d

In a 5 l reaction vessel with stirrer and reflux condenser, a mixture of1462.6 parts of the polyester diol A, 14.7 parts of neopentylglycol,89.4 parts of dimethylolpropionic acid and 450.9 parts oftetra-methylxylene diisocyanate are heated at a reaction temperature ofa) 95° C., b) 105° C., c) 115° C. and, respectively, d) 125° C. until aconstant NCO value of the mixture is reached. Subsequently, per mole ofdiisocyanate prepolymer, 1.78 times the quantity of moles oftrimethylolpropane are added and the mixture is allowed to react untilthe isocyanate content is virtually zero. The mixture is dissolved with924 parts of butoxypropanol and, in one portion at a temperature of 100°C., 55.7 parts of N,N-dimethylethanolamine are added. The mixture wasfinally adjusted to a solids content of 60% with butoxypropanol. TheGPCs were measured using tetrahydrofuran as mobile phase at a flow rateof 0.9 ml/min and an average pressure of 24 bar on a separating columnfrom Waters (Kombi) and polystyrene as standard.

Example 1a 1b 1c 1d Reaction temperature ° C.    95   105   115   125M_(n)  7,010  6,784  6,065  5,861 M_(w) 33,612 33,082 27,655 28,657

EXAMPLE 2 Preparation of the Polyurethane Resin Solution 2

In a 5 l reaction vessel with stirrer and reflux condenser, a mixture of1492.5 parts of the polyester diol B, 7.4 parts of neopentylglycol,106.7 parts of dimethylolpropionic acid and 447.4 [lacuna] oftetramethylxylene diisocyanate is heated to a reaction temperature of115° C. until a constant NCO value of the mixture is reached.Subsequently, per mole of diisocyanate, 1.88 times the quantity of molesof trimethylol propane are added and the mixture is allowed to reactuntil the isocyanate content is virtually zero. The mixture is dissolvedwith 500 parts of butoxypropanol and, in one portion at a temperature of100° C., 53.2 parts of dimethylethanolamine are added. The mixture wasfinally adjusted to a solids content of 60% with butoxypropanol (M_(n)5426, M_(w) 18532).

A solution of 10 parts of this resin with 3 parts of N-methylpyrrolidonehad a viscosity of 19 dpas (ICI plate/cone viscometer). The Gardnercolor number is 4-5.

Comparison Example 1 Preparation of the Polyurethane Resin Solution C1

In a 5 l reaction vessel with stirrer and reflux condenser, a mixture of995 parts of the polyester diol C, 4.9 parts of neopentylglycol, 71.2parts of dimethylolpropionic acid and 298.3 parts of tetramethylxylenediisocyanate is heated to a reaction temperature of 115° C. until aconstant NCO value of the mixture is reached. Subsequently, per mole ofdiisocyanate, 1.88 times the quantity of moles of trimethylolpropane areadded and the mixture is allowed to react until the isocyanate contentis virtually zero. The mixture is dissolved with 500 parts ofbutoxypropanol and, in one portion at a temperature of 100° C., 35.5parts of dimethylethanolamine are added. The mixture was finallyadjusted to a solids content of 60% with butoxypropanol (M_(w) [sic]5589, M_(w) 19062).

A solution of 10 parts of this resin with 3 parts of N-methylpyrrolidonehad a viscosity of 32 dPas (ICI plate/cone viscometer). The Gardnercolor number is 7-8.

EXAMPLE 3 Preparation of a Polyurethane Resin Dispersion

In a 5 l reaction vessel with stirrer and reflux condenser, a mixture of895.5 parts of the polyester diol B, 4.4 parts of neopentylglycol, 64.0parts of dimethylolpropionic acid and 268.5 parts of tetramethylxylenediisocyanate is heated at a reaction temperature of 115° C. until aconstant NCO value of the mixture is reached. Subsequently, per mole ofdiisocyanate, 1.88 times the quantity of moles of trimethylolpropane areadded and the mixture is allowed to react until the isocyanate contentis virtually zero. Following the addition of 38.2 parts ofN,N-dimethylethanolamine at 110° C., the mixture is stirred for half anhour. Deionized water is then added at 80° C. and the mixture isadjusted to a solids content of 35.8%. The pH of the dispersion was 7.4.The dispersion is stable at room temperature over a relatively longperiod.

Preparation of the Pigment-free Mixture B1

The following are added with stirring to 33.2 parts of the polyurethaneresin dispersion of Example 3: 43 parts of a preswollen aqueous pastecontaining 3% by weight of an inorganic sodium magnesium phyllosilicatethickening agent and 3% by weight of polypropylene glycol having anumber-average molecular weight of 900, the percentages being based onthe total weight of the paste, 19.8 parts of deionized water, 0.5 partof a commercially available antifoam and 3.5 parts of a 3.5% strengthsolution of a commercially available polyurethane thickener in water.

Preparation of a Blue-pigmented Base Color A1

12 parts of Paliogen Blue, 50 parts of the 60% strength neutralizedpolyurethane resin solution 1d, 23 parts of butylglycol and 15 parts ofisobutanol are mixed with stirring and dispersed using a sand mill.

Preparation of an Aluminum-containing Base Color A2

27 parts of an aluminum bronze chromatized in accordance with DE-A 3 636183 (aluminum content 65%, average particle diameter 15 μm) arehomogeneously dispersed in 6.6 parts of butylglycol, 23.6 parts of the60% strength polyurethane resin solution 1d and 13.6 parts of isobutanolby stirring for 15 minutes, and are then run with stirring into 29.2parts of the 60% strength neutralized polyurethane resin solution 1d.This mixture is stirred for a further 30 minutes using a high-speedstirrer at 100 rpm.

Preparation of Coating Compositions I to V

Polyurethane resin solution 1d and the polyurethane resin dispersion ofExample 3 are used to prepare aqueous coating compositions by stirringthe base color A1 or A2, respectively, directly after its preparationinto the mixture B1.

The viscosity is then adjusted to a flow time of about 25 sec. from aDIN-4 cup (20° C.).

TABLE Composition of the aqueous basecoat compositions I to V, in partsExample I II III IV V A2 19.00 14.25 12.7 9.5 6.3 A1 — 4.25 5.7 8.5 11.3B1 81.00 81.5 81.6 82.00 82.4 100.00  100.00 100.00 100.00 100.00

Directly after the preparation of the aqueous basecoat compositions theyare sprayed by well-known methods onto phosphatized steel panels (Bonder132) coated with a commercially available electrodeposition coating anda conventional (i.e. solvent-containing) or water-containing filler, andafter a flash-off time of 30 minutes at room temperature (at a relativeatmospheric humidity of 50% and a room temperature of 20° C.) are coatedover with a commercially available, conventional 2-component clearcoatbased on a hydroxyl group-containing acrylate copolymer and on anisocyanate crosslinking agent, and are dried at 60° C. for 30 minutes.The dry film thickness of the basecoat composition is ≈15 μm, that ofthe clearcoat ≈50 μm.

The coatings obtained correspond to those described in DE-A 41 10 520 inrespect of the metallic effect, the adhesion and the freedom fromclouding.

What is claimed is:
 1. Process for the preparation of polyurethaneresins, comprising reacting in ethoxyethyl propionate as solvent, a) atleast one compound having 2 functional groups selected from the groupconsisting of hydroxyl groups, amino groups, and mixtures thereof, b) atleast one compound selected from the group consisting of diisocyanates,polyisocyanates, and mixtures thereof, c) at least one compoundcontaining 2 groups which are reactive toward isocyanate groups, thesecompounds containing, at least in part, at least one group capable offorming anions, wherein components a), b) and c) are reacted to form anintermediate which has terminal isocyante groups and subsequentlyreacting at least some of the isocyanate groups with d) at least onecompound having at least 3 hydroxyl and/or amino groups.
 2. Processaccording to claim 1, characterized in that the resulting reactionproduct is transferred to an aqueous phase.
 3. Process according toclaim 2, characterized in that butoxypropanol is added as cosolvent. 4.Process according to claim 1 or 2, characterized in that the ethoxyethylpropionate is employed in a quantity of from 50 to 5% by weight based onthe solids content of the polyurethane resin solution.
 5. Processaccording to claim 1, characterized in that components (a), (b) (c) and(d) are reacted at a reaction temperature of from 80 to 150° C. 6.Coating composition based on polyurethane resins, characterized in thatit contains as binder a polyurethane resin prepared in accordance withclaim
 1. 7. Process for the production of coating compositionscomprising mixing a polyurethane resin prepared in accordance with claim1 with additional components selected from the group consisting ofpigments, additives, additional binders and mixtures thereof.
 8. Processfor the production of a multilayer protective and/or decorative coatingon a substrate surface, comprising 1.) applying a basecoat compositionto a substrate, 2.) forming a polymer film on the substrate surface fromthe composition applied in stage (1), 3.) applying a transparent topcoatto the resulting basecoat, and 4.) curing the topcoat together with thebasecoat, characterized in that the basecoat composition applied instage (1) and/or the topcoat applied in stage (3) contains apolyurethane resin which has been prepared in accordance with claim 1.9. Substrate coated with a single-layer or multilayer protective and/ordecorative coating, characterized in that at least one of the layers ofthe coating contains a polyurethane resin prepared according to claim 1.10. Water-dilutable coating composition comprising polyurethane resinsprepared by the process according to one claim
 1. 11. Automotivewater-dilutable coating composition comprising polyurethane resinsprepared by the process according to claim 1 as binder.
 12. Process forthe preparation of polyurethanes and aqueous polyurethane dispersionscomprising forming a polyurethane In the presence of ethoxyethylpropionate as a viscosity regulating, nonreactive component by reactinga) at least one compound having 2 functional groups selected from thegroup consisting of hydroxyl groups, amino groups, and mixtures thereof,b) at least one compound selected from the group consisting ofdilsocyanates, polyisocyanates, and mixtures thereof, c) at least onecompound containing 2 groups which are reactive toward Isocyanategroups, these compounds containing, at least in part, at least one groupcapable of forming anions, wherein components a), b), and c) are reactedto form an intermediate that has terminal isocyanate groups andsubsequently reacting at least some of the Isocyanate groups with d) atleast one compound having at least 3 hydroxyl and/or amino groups. 13.Process according to claim 2, characterized in that the resultingreaction product is transferred to an aqueous phase and further solventsare added as cosolvents.
 14. Process according to claim 1 or 2,characterized in that the ethoxyethyl propionate is employed in aquantity of less than 20% by weight, based on the solids content of thepolyurethane resin solution.
 15. Process according to claim 1,characterized in that components (a), (b), (c) and (d) are reacted at areaction temperature of from 100° C. to 120° C.
 16. Automotive refinishwater-dilutable coating composition comprising polyurethane resinsprepared by the process according to claim 1 as binder.